1. Field of the Invention
The present invention relates to a method of controlling the transmission power in a cellular system wherein a plurality of mobile stations communicate bidirectionally with each other via a base station situated in a service area, and a base station apparatus in such a base station.
2. Description of the Related Art
In a cellular system which employs a code division multiple access (CDMA) process for radio communications, a number of base stations and mobile stations communicate bidirectionally with each other in respective established links at the same frequency. The received power (desired power) of a signal in a certain link acts as interference power that disturbs other links. In an uplink transmission where a mobile station transmits a signal and a base station receives the transmitted signal, when the desired power exceeds a predetermined power level, the interference power increases, thus reducing the link capacity.
To prevent the above phenomenon from occurring, it is necessary to strictly control the transmission power of mobile stations. According to one uplink transmission power control process, the desired power from a mobile station is measured by a base station and the measured level of the desired power is compared with a control target level. If the measured level of the desired power is greater than the control target level, then the base station transmits a control command for reducing the transmission power to the mobile station. Conversely, if the measured level of the desired power is smaller than the control target level, then the base station transmits a control command for increasing the transmission power to the mobile station. This transmission power control process is discussed in detail in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod and apparatus for controlling the transmission power in a CDMA cellular systemxe2x80x9d, issued to Gilhousen et al.
According to the above transmission power control process, however, if a control command transmitted from a base station to a mobile station is judged erroneously so as to increase or reduce the transmission power in a manner opposite to the control command, then the desired power may become excessive or insufficient in level. If the desired power becomes excessive, then the interference power which affects other links increases. If the desired power becomes insufficient, then the quality in the link decreases.
Japanese unexamined patent publication No. 1997-312609 discloses a process of ignoring a received control command if the quality of the received control command is poor, so that the transmission power from a mobile station will be prevented from increasing or decreasing in a wrong direction for thereby alleviating the above drawback.
Code division multiple access (CDMA) cellular systems use a technique known as xe2x80x9csoft handoffxe2x80x9d which allows a mobile station, as it moves from one cell to another, to communicate with a plurality of base stations in the vicinity of the boundary of the cells while changing links. The soft handoff is disclosed in detail in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMethod and system for providing a soft handoff in communications in a CDMA cellular telephone systemxe2x80x9d, issued to Gilhousen et al.
According to an uplink transmission power control process with a soft handoff function, a plurality of base stations measure the desired power from a mobile station, and independently transmit respective control commands based on the measured levels of the desired power to the mobile station via downlinks. When the mobile station receives the control commands from the respective base stations, the mobile station controls the transmission power based on the received control commands. If the mobile station receives different control commands, then the mobile station controls the transmission power based on the control command for reducing the transmission power. This process is described in TIA/EIA Interim Standard, Mobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95-A, Telecommunication Industry Association, May 1995, 6.6.6.2.7.2 Reverse Traffic Channel Power Control During Soft Handoff.
As described above, if the mobile station receives different command signals transmitted from the respective base stations, then the mobile station controls the transmission power on the basis of the control command for reducing the transmission power, from among the control commands transmitted from the respective base stations and received by the mobile station. Therefore, the desired power does not exceed a control target level in any of the base stations, for thereby achieving a high uplink capacity. In this process, consequently, it is important to reduce the probability that the mobile station will fail to receive a control command for reducing the transmission power.
In a downlink transmission where a base station transmits a signal and a mobile station receives the transmitted signal, the transmission power from the base station is controlled to achieve a predetermined ratio between the desired power and the interference power for thereby accomplishing a high link capacity.
According to the uplink transmission power control, as described above, the base stations independently transmit respective control commands for controlling the transmission power to the mobile station via downlinks. It is important that, while the soft handoff function is being performed, the downlink transmission power be controlled in order for the mobile station to be able to receive control commands transmitted from the base stations.
One solution is to control at the mobile station the downlink transmission power levels for equalizing respective desired power levels from the base stations. According to this solution, in those base stations which suffer a large downlink propagation loss, the transmission power is set to a correspondingly large level. Therefore, the interference power increases, resulting in a reduction in the downlink capacity. Such a reduction in the downlink capacity can be suppressed by controlling the transmission power levels so as to equalize transmission power levels from the base stations, as disclosed in Anderson, xe2x80x9cTuning the macro diversity performance in a DS-CDMA system,xe2x80x9d Proc. IEEE 44th Vehicular Technology Conference, pp. 41-45, June 1994.
According to the above transmission power control scheme, however, the desired power levels from the base stations which suffer a large downlink propagation loss are so reduced that the mobile station is more likely to fail to receive control commands from those base stations and hence it is more probable that the mobile station will fail to receive control commands to reduce the transmission power.
Instantaneous level variations of the propagation loss differ from frequency to frequency. In a system where different frequencies are employed for uplink and downlink transmissions, uplink and downlink propagation losses differ from each other. Therefore, on the assumption that an uplink propagation loss from a mobile station to a base station is small and the desired power received by the base station is greater than a control target level, when a control command for reducing the transmission power is transmitted from the base station to the mobile station, if a downlink propagation loss from the base station to the mobile station is larger, then the desired power received by the mobile station is smaller, with the result that the mobile station may possibly fail to receive the transmitted control command for reducing the transmission power.
If all control commands transmitted from other base stations to the mobile station at this time are for reducing the transmission power, then the transmission power is increased by the mobile station, and the desired power in the base station which has transmitted the control command that the mobile station has failed to receive becomes excessive. Consequently, the interference power will be greater than if the mobile station had successfully received the control command and controlled the transmission power accordingly, resulting in a reduction in the uplink capacity of the base station.
According to the above solution to control the downlink transmission power levels for equalizing respective desired power levels from the base stations in the mobile station, therefore, the possibility that the base station will be able to receive all control commands transmitted from the base stations is lowered, resulting in a reduction in the uplink capacity. According to a process disclosed in Japanese unexamined patent publication No. 1997-261170, the above problem can be solved as follows: When a control command for reducing the transmission power of a mobile station is transmitted from a base station, the transmission power of the control command transmitted from the base station is set to a level higher than the levels of other commands, so that the mobile station will be able to receive, with a greater probability, the control command from the base station.
While the soft handoff function is being performed, it is of importance that a mobile station control the transmission power according to a control command transmitted from a base station with a low propagation loss so that the desired power in the base station will be equalized to a control target level. However, on the assumption that the mobile station fails to receive a control command transmitted from a base station with a high propagation loss and recognizes a control command for increasing the transmission power erroneously as a control command for reducing the transmission power, then even if a control command transmitted from a base station with a low propagation loss is for increasing the transmission power, the mobile station preferentially controls the transmission power based on the control command for reducing the transmission power, and hence the base station suffers a shortage of the desired power and a reduction in the link quality. However, the process disclosed in Japanese unexamined patent publication No. 1997-312609 is capable of lessening the above difficulty because it ignores a received control command if the quality of the received control command is poor, and is less adversely affected by errors in determining control commands of low reliability from base stations with a high propagation loss.
The invention disclosed in Japanese unexamined patent publication No. 1997-261170, as it is related to the present invention, will be described below with reference to FIGS. 1 through 6 of the accompanying drawings.
FIG. 1 of the accompanying drawings is a flowchart of a process for a base station to receive a frame error rate reported from a mobile station, determine a downlink transmission power level, and transmit a control command for an uplink transmission power level.
As shown in FIG. 1, when the base station is to start communicating with the mobile station, the base station sets the downlink transmission power Pf which is determined depending on a frame error rate to an initial level Pf0 in step S1201. The frame error rate is reported at given intervals from the mobile station.
If there is a newly reported frame error rate in step S1202 and the newly reported frame error rate is greater than a predetermined range in step S1203, then the base station increments the downlink transmission power Pf by xcex94P ( greater than 0) in step S1204. If the newly reported frame error rate is smaller than the predetermined range in step S1205, then the base station decrements the downlink transmission power Pf by xcex94P in step S1206.
If there is no newly reported frame error rate in step S1202, then the base station maintains the downlink transmission power Pf as it is. Thereafter, control goes to step S1207.
In step S1207, if a measured level D of uplink desired power which is measured in each frame is greater than a predetermined control target level T, then the base station determines a control command for reducing uplink transmission power in step S1208, and equalizes the downlink transmission power P to Pf+Padd in step S1209, where Padd represents a transmission power increment of predetermined constant value.
Conversely, if the measured level D of uplink desired power is smaller than the predetermined control target level T in step S1207, then the base station determines a control command for increasing the uplink transmission power in step S1210, and equalizes the downlink transmission power P to the downlink transmission power Pf which is determined depending on the frame error rate in step S1211.
The base station then transmits the control command for the uplink transmission power to the mobile station with the downlink transmission power P in step S1212. Thereafter, control repeats the processing from step S1202.
A mobile station which is establishing a link with a single base station controls the uplink transmission power according to a control command that is transmitted from the base station for the uplink transmission power. A mobile station which is simultaneously establishing links with two base stations receives two control commands that are transmitted from the respective base stations for the uplink transmission power. If the control commands are different from each other, then the mobile station controls the uplink transmission power according to the control command for reducing the uplink transmission power.
According to the above transmission power control process shown in FIG. 1, while the soft handoff function is being performed, the base stations, i.e., main and auxiliary base stations, receive the same frame error rate which has been reported and similarly increase or reduce the downlink transmission power Pf which is determined depending on the frame error rate. Therefore, the downlink transmission power Pf is equalized by the base stations. When the base stations are to transmit control commands for reducing the uplink transmission power to the mobile station, the base stations establish the downlink transmission power P so as to be greater than the downlink transmission power Pf, determined depending on the frame error rate, by the transmission power increment Padd. The transmission power control process is thus capable of reducing the probability that the mobile station fails to receive a control command for reducing the uplink transmission power, and the uplink excessively interferes with the other links.
FIGS. 2A and 2B of the accompanying drawings illustrate a flowchart of a process for a base station to receive the reception power Q of a pilot signal, measured in each frame, reported from a mobile station, determine a downlink transmission power level, and transmit a control command for an uplink transmission power level.
As shown in FIGS. 2A and 2B, when the base station is to start communicating with the mobile station, the base station sets the downlink transmission power Pf which is determined depending on a frame error rate to an initial level Pf0 in step S1301. The frame error rate is reported at given intervals from the mobile station.
If there is a newly reported frame error rate in step S1302 and the newly reported frame error rate is greater than a predetermined range in step S1303, then the base station increments the downlink transmission power Pf by xcex94P ( greater than 0) in step S1304. If the newly reported frame error rate is smaller than the predetermined range in step S1305, then the base station decrements the downlink transmission power Pf by xcex94P in step S1306.
If there is no newly reported frame error rate in step S1302, then the base station maintains the downlink transmission power Pf as it is. Thereafter, control goes to step S1307.
In step S1307, if a measured level D of uplink desired power which is measured in each frame is greater than a predetermined control target level T, then the base station determines a control command for reducing uplink transmission power in step S1308. If the reception power Q of a pilot signal of its own station is not maximum in step S1309, then the base station equalizes the downlink transmission power P to Pf+Padd in step S1310, where Padd represents a transmission power increment of predetermined constant value.
If the reception power Q of the pilot signal is maximum in step S1309, then the base station equalizes the downlink transmission power P to the downlink transmission power Pf which is determined depending on the frame error rate in step S1312.
If the measured level D of uplink desired power is smaller than the predetermined control target level T in step S1307, then the base station determines a control command for increasing the uplink transmission power in step S1311, and thereafter equalizes the downlink transmission power P to the downlink transmission power Pf which is determined depending on the frame error rate in step S1312.
The base station then transmits the control command for the uplink transmission power to the mobile station with the downlink transmission power P in step S1313. Thereafter, control repeats the processing from step S1302.
According to the above transmission power control process shown in FIGS. 2A and 2B, when a base station is to transmit a control command for reducing uplink transmission power to a mobile station, only if the mobile station is highly likely to fail to receive the control command for reducing uplink transmission power unless downlink transmission power P is increased because the reception power Q of a pilot signal of its own station is not maximum, then the base station increases the downlink transmission power P by a transmission power increment Padd.
Therefore, the probability that excessive uplink interference is caused due to a failure by the mobile station to receive a control command for reducing uplink transmission power is reduced, and the interference power is prevented from increasing owing to an increase in the downlink transmission power P.
FIG. 3 of the accompanying drawings is a flowchart of a process of determining a transmission power increment Padd in a conventional transmission power control process.
As shown in FIG. 3, a base station receives reception power Q of pilot signals of main and auxiliary base stations reported from a mobile station in step S1401.
The base station regards the reception power of a pilot signal of its own station, reported from the mobile station, as Q0, in step S1402, and also regards the reception power of pilot signals of other base stations, reported from the mobile station, as Qi (i=1xe2x88x92Nxe2x88x921 where N is the sum of the number of main base stations and the number of auxiliary base stations) in step S1403.
Then, the base station sets a counter i to 1 in step S1404, and sets the maximum level Qmax for the reception power of pilot signals to Qi in step S1405.
If the counter i is not the same as (Nxe2x88x921) in step S1406, then the base station increments the counter i by 1 in step S1407. If the reception power Qi of pilot signals is greater than the maximum level Qmax in step S1408, then control goes back to step S1405, and the maximum level Qmax is set to Qi.
If the reception power Qi of pilot signals equal to or smaller than the maximum level Qmax in step S1408, then control goes back to step S1406.
If the counter i is the same as (Nxe2x88x921) in step S1406, then the base station sets the transmission power increment Padd to (Qmaxxe2x88x92Q0) in step S1409. Thereafter, the process shown in FIG. 3 comes to an end.
In this manner, the maximum level Qmax for the reception power Qi of pilot signals is searched for, and the transmission power increment Padd is determined as Padd=(Qmaxxe2x88x92Q0).
According to the process shown in FIG. 3, when a base station is to transmit a control command for reducing uplink transmission power to a mobile station at the time the reception power Q0 of a pilot signal of its own station is not maximum, the base station establishes downlink transmission power P so as to be greater than with respect to the base station whose pilot signal reception power Qi is maximum, by the reduction in the reception power Q0 of the pilot signal of its own station, i.e., the increase in the propagation loss. Therefore, a measured level D of desired power in the mobile station becomes the same as the levels of desired power which would be achieved by control commands for uplink transmission power from other base stations.
Inasmuch as the downlink transmission power P is increased by a level necessary for lowering the probability that the mobile station fails to receive a control command for reducing the uplink transmission power, the downlink interference power is further prevented from increasing.
FIG. 4 of the accompanying drawings is a flowchart of a process for an exchange to receive a measured level D of desired power which is indicated, determine a control command for uplink transmission power, and send the determined control command to a base station.
As shown in FIG. 4, an exchange receives measured levels Di (i=0xe2x88x92Nxe2x88x921) of desired power reported from respective base stations in step S1501.
The exchange also receives control target levels Ti (i=0xe2x88x92Nxe2x88x921) of desired power reported from the respective base stations in step S1502. Here, it is assumed that the control target levels Ti of desired power are of a constant value common to all the base stations.
Then, the exchange sets a counter i to 0 in step S1503. If the difference (Dixe2x88x92Ti) between the measured level Di of desired power of the ith base station and the control target level Ti of desired power of the ith base station is not greater than 0 in step S1504, then the exchange compares the value of the counter i with (Nxe2x88x921) in step S1505. If the value of the counter i is not equal to (Nxe2x88x921), then the exchange increments the value of the counter i with 1 in step S1506, and repeats the processing from step S1504.
If the value of the counter i is equal to (Nxe2x88x921) in step S1505, then the exchange determines a control command for increasing uplink transmission power in step S1507.
If the difference (Dixe2x88x92Ti) is greater than 0 in step S1504, then the exchange determines a control command for reducing uplink transmission power in step S1508.
Thereafter, the exchange sends the determined control command for uplink transmission power to each of main and auxiliary base stations in step S1509.
Each of the base stations sends the control command for uplink transmission power from the exchange via the downlink to a mobile station, which then controls uplink transmission power according to the supplied control command.
According to the process shown in FIG. 4, since the same control command for uplink transmission power is transmitted from the main and auxiliary base stations, insofar as the mobile station successfully receives at least one control command, even if it fails to receive other control commands, the base station can control uplink transmission power to achieve required desired power without imposing excessive interference power on any other base stations.
As a consequence, the uplink capacity can be increased.
FIG. 5 of the accompanying drawings is a flowchart of a process for an exchange to receive a temporary control command indicated by a base station, determine a control command for uplink transmission power, and send the determined control command to the base station.
As shown in FIG. 5, an exchange receives a temporary control command from each of respective base stations with respect to each mobile station in step S1601.
If at least one of the received temporary control commands is for reducing transmission power in step S1602, then the exchange determines a control command for reducing uplink transmission power in step S1603. If all the received temporary control commands are for increasing transmission power in step S1602, then the exchange determines a control command for increasing uplink transmission power in step S1604.
Thereafter, the exchange sends the determined control command for uplink transmission power to each of the main and auxiliary base stations in step S1605.
Each of the base stations sends the control command for uplink transmission power from the exchange via the downlink to a mobile station, which then controls uplink transmission power according to the supplied control command.
According to the process shown in FIG. 5, since the same control command for uplink transmission power is transmitted from the main and auxiliary base stations, insofar as the mobile station successfully receives at least one control command, even if it fails to receive other control commands, the base station can control excessive interference power on any base stations.
As a consequence, the uplink capacity can be increased.
FIG. 6 of the accompanying drawings is a flowchart of a process for a base station to determine whether or not a base station is to transmit a signal while a soft handoff function is being performed.
As shown in FIG. 6, a base station receives reception power Q of pilot signals of main and auxiliary base stations from a mobile station in step S1701.
The base station regards the reception power of a pilot signal of its own station, reported from the mobile station, as Q0, in step S1702, and also regards the reception power of pilot signals of other base stations, reported from the mobile station, as Qi (i=1xe2x88x92Nxe2x88x921 where N is the sum of the number of main base stations and the number of auxiliary base stations) in step S1703.
Then, the base station sets a counter i to 1 in step S1704, and sets the maximum level Qmax for the reception power of pilot signals to 0 in step S1705.
If the reception power Qi of pilot signals is greater than the maximum level Qmax in step S1706, then the base station sets the maximum level Qmax to Qi in step S1707. The base station compares the counter i with (Nxe2x88x921). If the counter i is not equal to (Nxe2x88x921) in step S1708, then the base station increments the counter i by 1 in step S1709. Then, control repeats the processing from step S1706.
If the reception power Qi is equal or smaller than the maximum level Qmax in step S1706, then control jumps to step S1708.
If the counter i is equal to (Nxe2x88x921) in step S1708, then control goes to step S1710 in which the base station compares the maximum level Qmax with the reception power Q0 of the pilot signal of its own station. If the reception power Q0 is equal to or greater than the maximum level Qmax, then its own base station transmits a downlink signal in step S1711.
If the reception power Q0 is smaller than the maximum level Qmax in step S1710, other base stations transmit a downlink signal, but its own base station does not transmit a downlink signal.
As described above, while a soft handoff function is being performed, each base station receives reception power Q of all pilot signals of main and auxiliary base stations from a mobile station. If the reception power Q0 of the pilot signal of its own station is maximum, then the base station transmits a control command for uplink transmission power from an exchange via the downlink to the mobile station, and the mobile station controls uplink transmission power according to the transmitted control command for uplink transmission power.
According to the above transmission power control process, it is possible to control uplink transmission power to achieve required desired power without imposing excessive interference power on any base stations even in a cellular system in which only one of main and auxiliary base stations transmits a downlink signal based on the reception power Q of the pilot signal in the mobile station.
As a consequence, the uplink capacity can be increased.
In the conventional cellular system described above, unless downlink transmission power is adequately controlled, a mobile station is unable to properly receive a control command for controlling transmission power, which is transmitted from a base station via the downlink, resulting in excessive uplink desired power and increased interference power.
In cases where a mobile station controls transmission power based on a control command whose reception quality is good while ignoring a control command whose reception quality is poor, control commands may successively be judged as suffering poor reception quality depending on the criterion established for determining the reception quality, and the mobile station may be prevented from controlling transmission power, causing the same problems as described above.
In the process for increasing transmission power from a base station when a control command for reducing transmission power is transmitted from the base station to a mobile station while a soft handoff function is being performed, there may be occasions where the mobile station receives the control command with good quality even though the transmission power from the base station is not increased. In those occasions, since the control command is transmitted from the base station with more transmission power than required, downlink interference power is increased.
In the process for ignoring a control command whose reception quality is poor while a soft handoff function is being performed, the probability that a control command transmitted from a base station whose downlink propagation loss is larger is ignored is higher than the probability that a control command transmitted from a base station whose downlink propagation loss is smaller is ignored. If this process is applied to a cellular system where different frequencies are employed for uplink and downlink transmissions, then a certain base station may have a smaller uplink propagation loss than other base stations and receive excessive desired power because instantaneous uplink and downlink level variations differ from each other. In this case, even though a control command for reducing transmission power is continuously transmitted from the base station, if the base station has a larger downlink propagation loss than other base stations, then the control command transmitted from the base station to a mobile station is continuously ignored, and transmission power in the mobile station is controlled according to a control command transmitted from another base station whose uplink propagation loss is greater. In the base station whose uplink propagation loss is smaller, desired power remains excessive, and uplink interference power is increased. Particularly if the mobile station moves at a low speed and hence undergoes slow instantaneous level variations, the increased interference power remains continuously.
Even if the above process of ignoring certain control commands is not employed, control commands whose reception quality is poor may fail to be received or may be judged in error.
It is therefore an object of the present invention to provide a method of controlling transmission power in a cellular system to prevent signals from being transmitted from a mobile station to a base station with excessive transmission power for thereby increasing uplink capacity, and a base station apparatus for use in such a base station.
If, in a mobile station which has a link established with a base station, transmission power has been controlled on the basis of a first control command transmitted from the base station, then the reception quality of a signal transmitted from the mobile station, which is measured in the base station, is of a level close to target reception quality. If, in the mobile station, transmission power has been controlled on the basis of a first control command transmitted from a plurality of base stations, then the reception quality of a signal transmitted from the mobile station, which is measured in the base station, is of a level close to target reception quality or a level smaller than target reception quality.
According to the present invention, the reception quality of a signal transmitted from a mobile station is compared with target reception quality, and an increment for transmission power which is determined according to another algorithm is determined on the basis of a compared result. A control command is transmitted with transmission power which has been increased by the increment from the transmission power according to the other algorithm. If the increment determined according to the other algorithm is reduced, then the transmission power with which to transmit the control command is returned to the transmission power according to the other algorithm, thus lowering the probability that the mobile station fails to receive a control command for reducing transmission power. Therefore, the advantage of the other algorithm for transmission power control can also be attained.
With the above arrangement of the present invention, the reception quality of a signal transmitted from a mobile station is compared with target reception quality, and an increment for transmission power of a base station is determined on the basis of a compared result. The increment is added to the transmission power of the base station based on a second control command transmitted from the mobile station, and a first control command for controlling transmission power of the mobile station is transmitted with the transmission power with the increment added thereto. If the reception quality of the signal transmitted from the mobile station increases in excess of a predetermined range of target reception quality, then the increment for the transmission power is increased. If the reception quality of the signal transmitted from the mobile station falls within or becomes smaller than the predetermined range of target reception quality, then the increment for the transmission power is reduced. When the reception quality of the signal transmitted from the mobile station thus becomes close to or smaller than the target reception quality, the transmission power of the base station is returned to transmission power determined according to another algorithm.
While the transmission power is being controlled according to the an other algorithm, the probability that the mobile station will fail to receive a control command for reducing transmission power is lowered, and interference power is prevented from increasing.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.